The present technique relates generally to the control of induction machines and, more particularly, to the control of induction machines via field oriented control techniques.
Induction machines, such as motors and generators, are commonly found in industrial, commercial, and consumer settings. In industry, such machines are employed to drive various kinds of devices, including pumps, conveyors, compressors, fans, and so forth, to mention only a few, as well as for the generation of power. In the case of electric motors and generators, these devices generally include a stator, comprising a multiplicity of stator windings, surrounding a rotor.
By establishing an electromagnetic relationship between the rotor and the stator, electrical energy can be converted into kinetic energy, and vice-versa. In alternating current (ac) motors, ac power applied to the stator windings effectuates rotation of the rotor. The speed of this rotation is a function of the frequency of the ac input power (i.e., frequency) and of the motor design (i.e., the number of poles defined by the stator windings). Advantageously, a rotor shaft extending through the motor housing takes advantage of this produced rotation, translating the rotor's movement into a driving force for a given piece of machinery. Conversely, in the case of an ac generator, rotation of an appropriately magnetized rotor induces current within the stator windings, in turn producing electrical power.
Control of such induction machines may be conducted in accordance with field-oriented-control or vector control techniques. In summary, field-oriented-control techniques are used to control the speed and torque of an ac motor by resolving the ac current feed to the stator into a torque-producing current (iq) and a flux-producing current (id). Such vector analysis allows an induction machine to be viewed as a direct current (dc) device, where field current controls the flux in the device and armature current controls the torque in the device. Descriptions of field-oriented-control schemes are provided in U.S. Pat. No. 5,032,771 that issued on Jul. 16, 1991, to Kerkman et al., and U.S. Pat. No. 5,717,305 that issued on Feb. 10, 1998, to Seibel et al., and both of these patents are incorporated herein by reference. Thus, in traditional field-oriented-control systems, torque control of the motor is effectuated by varying the iq vector component, while the id vector component remains constant.
However, in many instances, such as in flywheel based uninterruptible power source (UPS) systems—an example of which is described in U.S. patent application Ser. No. 10/944,064, which was filed on Sep. 17, 2004, is entitled “APPARATUS AND METHOD FOR TRANSIENT AND UNINTERRUPTIBLE POWER,” and is incorporated herein by reference-less torque is required to maintain continued operation of the device than is required during start-up or loaded conditions. Keeping in mind the relationship between torque and flux, if less torque is required to maintain operational speed, then flux may be lessened as well. Indeed, at low torque requiring conditions, flux vector components levels can be reduced while still maintaining desired operational speeds.
Unfortunately, in traditional field-oriented-control techniques the invariance of the id vector component translates into a usage of current that is higher than necessary when the torque required is low. In turn, traditional field-oriented-control techniques draw excess current and excess power, leading to increased costs based on power consumption. Moreover, drawing excess current and power increases resistive heating, hysteresis, and eddy current losses in the stator, all of which are undesirable.
Therefore, there exists a need for improved field-oriented-control techniques.